mwl8k: fix possible race condition in info->control.sta use

info->control.sta may only be dereferenced during the drv_tx call otherwise
could lead to use-after-free bugs

Reported-by: Felix Fietkau <nbd@nbd.name>
Signed-off-by: Thomas Huehn <thomas@net.t-labs.tu-berlin.de>
Signed-off-by: John W. Linville <linville@tuxdriver.com>